Frequency hopping (FH) is a well known spread spectrum technique. Frequency hopping can be used as a multiple access technique in order to share a communications resource among numerous user groups. Since a user group typically employs a unique spread spectrum signaling code, (e.g., frequency hopping set) privacy between individual user groups is easily established.
Local area networks use spread spectrum signaling techniques where a series of user devices, which are typically battery powered, communicate with the access point (AP) which functions as a central controller and may also act as an information relay device. A radio transceiver or wireless adapter within each user device and within the AP provides the radio communication function. The AP typically provides network services such as synchronization, authentication, wireline access, packet relay between user devices, and the like. Communication between user devices may be accomplished directly between user devices or may be accomplished by relaying information from a source user device to the AP and back to a destination user device. The group of user devices and the corresponding AP are referred to as a microcell.
Microcells may also exist that do not have an AP. Such systems are referred to as ad-hoc microcells. For example, a microcell may consist of several portable computers that form a temporary network established for the duration of a meeting in a conference room. In this case, one of the user devices is designated as a master device and is responsible for synchronization between the other user devices.
In a frequency hopping LAN, all devices within one microcell share the same hopping sequence. Each user device changes receiver frequency in unison with all other user devices within the microcell and in unison with the AP such that the change in frequency is virtually transparent to the user devices. The time during which a device is tuned to an individual frequency of the hopping set is referred to as a dwell. The dwell is typically long enough to allow several transmissions of data, referred to as packets, to be transmitted to or from the user devices or the AP.
Many office environments have several microcells operating in close proximity to each other and may be used to provide extended coverage over a large area. In such a situation, microcells must not interfere with neighboring microcells and must facilitate transparent link transfers therebetween. When a user device seeks or requires access to the AP of a different microcell, a link transfer or "hand-off" must be performed between the two microcell controllers (e.g., APs 14). In most cases, however, neighboring microcells employ completely different spread-spectrum signaling codes or hopping sequence sets. As a consequence, acquisition and synchronization are formidable challenges during a hand-off.
If a hand-off is too slow the network may experience various time-outs which may in turn cause the user's session to be dropped, resulting in disconnection from the network. Some systems may even lock-up or experience serious system performance degradation when this occurs.
Typical wireless LANs and devices associated therewith may use a channel access protocol such as carrier sense multiple access (CSMA) to determine a time during which to transmit information, as is known in the art. When two devices transmit at the same time, a collision occurs and neither transmission is successful. The purpose of the CSMA protocol is to allocate the communication channel fairly among the devices to maximize information throughput.
Other LANs use a channel access protocol such as carrier sense multiple access with collision avoidance (CSMA/CA) to determine a time during which to transmit information. This technique allows the devices to "sense" if a channel is in use prior to transmission. If the channel is in use or is busy, transmission is deferred to another time according to specific collision avoidance algorithms.
The user devices and in particular, the wireless adapter portion, consume significant quantities of power, and hence, affects the battery life of the user device. The wireless adapters typically utilize a power management strategy to conserve power. Under one typical power management strategy the wireless adapter alternates between a radio "sleep" state and a radio "awake" state. In the sleep state the device can neither receive nor transmit and, hence, cannot participate in communication activity within the microcell.
As a device sleeps more frequently, less power is consumed. However, the ability of the device to communicate and maintain synchronization with the microcell will decrease as the sleep time increases. Typical power management strategies attempt to balance power consumption and channel access performance.
The user devices and the AP transmit information in the form of data packets as described above. Packets fall into three categories based upon destination: 1) unicast packet, 2) multicast packet, and 3) broadcast packet. A unicast packet is destined for a single specific address or single user device. A multicast packet is destined for more than one address or user device, but not necessarily all user devices. A broadcast packet is destined for all addresses or user devices where the broadcast packet is a superset of the multicast packet.
Typical communication systems employ a strategy for acknowledging the reception of unicast data packets. After a source user device transmits the unicast packet, a destination device transmits an acknowledgment back to the source user device. Upon receipt of the acknowledgment, the source user device will know that its packet transmission was successful. Multicast and broadcast packets, however, are typically not acknowledged by the destination user device since the number of subsequent acknowledgments returned would degrade system performance. Since no acknowledgments are returned after delivering multicast or broadcast packets, such transmissions are not as reliable as unicast packet transmissions.
In known wireless communication systems wireless devices are usually required to be within communicating range of the AP. Such a requirement may be enforced by the registration and synchronization procedures in effect within the LAN or microcell. However, there is no guarantee that a given user device will be within range of another wireless user device. Thus, transmission between two user devices may be unsuccessful.
Additionally, packet delivery between two user devices is further complicated by the fact that the destination user device might either be asleep or out of range of the source user device when the transmission occurs. It would be extremely advantageous therefore to provide a method for delivering multicast and broadcast packets to all devices in the microcell regardless of whether some devices are out of range of each other. It would also be advantageous to provide a method for allowing user devices to save battery power by scheduling sleep and awake states that allow the user device to reliably receive multicast and broadcast packets.